从 Python 第三方进度条库 tqdm 谈起 （转载）

原文地址：

https://blog.ernest.me/post/python-progress-bar

tqdm

最近一款新的进度条 tqdm 库比较热门，声称比老版的 python-progressbar 库的单次响应时间提高了 10 倍以上。

Overhead is low -- about 60ns per iteration (80ns with gui=True). By comparison, the well established ProgressBar has an 800ns/iter overhead.

初读其源码，组织结构明显继承 python-progressbar，只是主代码行数从 357 提升到了 614。10 倍性能提升的奥妙在哪里呢？
在解答这个问题之前，我想先用这篇文章介绍下进度条的原理，然后，根据原理用几行代码实现一个简单的进度条。

progress bar 的原理

其实进度条的原理十分的简单，无非就是在 shell 中不断重写当前输出。
这时就不得不提到文本系统中的控制符。我们挑跟这次有关的看一下。

• = CR (Carriage Return) // moves the cursor to the beginning of the line without advancing to the next line（该控制符告诉输出端，将光标移到当前行的首位而不换行）
• = LF (Line Feed) // moves the cursor down to the next line without returning to the beginning of the line - *In a nix environment moves to the beginning of the line.（传统意义上的换行符，将光标移到下一行，但_并不移到首位_ ）
• = CR + LF // a combi of and （换行并移动光标到行首）

这时，想要实现一个进度条，就十分简单，看下方代码。

 

 

Bash 实现

#/usr/bin/bash
for i in {1..100};
do
    echo -ne "$i% 
"
    sleep 0.01
done
echo -ne "
"

但是，echo -n 存在明显的兼容性问题。

-n Do not print the trailing newline character. This may also be achieved by appending c to the end of the string, as is done by iBCS2 compatible systems. Note that this option as well as the effect of c are implementation-defined in IEEE Std 1003.1-2001 (POSIX.1) as amended by Cor. 1-2002. Applications aiming for maximum portability are strongly encouraged to use printf(1) to suppress the newline character.

Some shells may provide a builtin echo command which is similar or identical to this utility. Most notably, the builtin echo in sh(1) does not accept the -n option. Consult the builtin(1) manual page.

推荐使用 printf。

#/usr/bin/bash
for i in {1..100};
do
    printf "%s%% 
" $i
    sleep 0.01
done
printf "
"

 

 

Python 实现

Python 主要使用系统库里的标准输出，sys.stdout 提供了便利的方法用于向 shell 打印输出。具体的方法介绍这里不赘述。

import sys
import time

for i in range(100):
    sys.stdout.write('   
')
    sys.stdout.flush()
    sys.stdout.write('{}%
'.format(i))
    sys.stdout.flush()
    time.sleep(0.01)

References

• tqdm repo
• progress bar repo
• NewLine Wikipedia
• Control Characters
• What's the difference between and